This project is a revised, competing renewal R01 application that focuses on the contribution of immune dysregulation to pulmonary arterial hypertension (PH). PH is a serious condition frequently associated with autoimmune conditions and perivascular inflammation. Regulatory T cells (Tregs) normally serve a protective function by promoting injury resolution through several anti-inflammatory mechanisms including the induction of anti-inflammatory macrophages. When Treg function is compromised, as it is in a number of PH-associated conditions, injury evolution occurs with inflammatory responses that are overly exuberant, notably with activated perivascular M1 macrophages. Accordingly, athymic rats which lack all T cell populations, including Tregs, develop severe perivascular inflammation and PH when treated with monocrotaline or with a vascular endothelial growth factor receptor 2 antagonist. PH, in these animals, is prevented if Tregs are restored through immune reconstitution; in these rescued rats, the hallmark of protection is an early influx of Tregs expressing the anti-inflammatory cytokine IL-10. Infiltrating IL-10+ Tregs in the lungs are associated with increased anti-inflammatory macrophages which secrete the immunomodulatory protein heme oxygenase 1 (HO-1). IL-10, a known inducer of HO-1, may be a key Treg-derived mediator promoting the appearance of anti-inflammatory Mox (HO-1+) macrophages. The absence of these Tregs in non-reconstituted athymic rats results in a significant inflammatory response with notable M1 macrophage infiltration, similar to clinical PH. Activated macrophages in experimental and clinical PH strongly express the leukotriene, LTB4, which mediates pulmonary artery endothelial cell (PAEC) apoptosis and PA smooth muscle cell (PASMC) proliferation and hypertrophy. Inhibiting macrophage-derived biosynthesis of LTB4 or blocking the LTB4 receptor, BLT1, prevents PAEC apoptosis, a key early event in PH pathogenesis. Accordingly, systemic, oral or inhaled LTB4 antagonists not only prevent PH but also reverse advanced disease. LTB4 mediates PAEC apoptosis through inhibition of the sphingosine kinase 1-endothelial nitric oxide (eNOS) survival pathway. Specific Aim 1 will be to determine whether pulmonary vascular injury resolution associated with the induction of anti-inflammatory macrophages requires IL-10 dependent Treg activity. Specific Aim 2 will be to determine how macrophage- derived LTB4 mediates PH-relevant vascular injury evolution. This aim will determine whether LTB4 inhibits endothelial pro-survival Smad/BMPR2 signaling through NF?B. Additionally, this aim will test whether macrophage-derived LTB4 blocks endothelial Sphk1-eNOS survival signaling through p53. Finally, Aim 2 will assess whether macrophage-derived LTB4 induces PASMC proliferation, hypertrophy, migration and survival in a BLT1-dependent manner. LTB4 antagonism may be a highly effective therapy for a subset of PH patients with immune dysregulation. This project integrates evolving concepts about pulmonary immunity with vascular biology to provide a better understanding of how the immune response contributes to health and disease.